1. Field of the Invention
The present invention relates to a radio frequency (RF) integrated circuit of a semiconductor device using a transformer instead of an inductor, and more particularly, to a Q-boosting circuit capable of enhancing a quality factor (Q).
2. Discussion of Related Art
Recently, as a mobile communication service is expanding fast and the size of a terminal is reduced, it is required to reduce the size of an inductance-capacitance (LC) resonator, used in an RF integrated circuit such as a voltage-controlled oscillator (VCO).
When an inductor is integrated, the Q factor of the inductor considerably deteriorates. Thus, in order to solve this problem, a method that embodies an inductor using a transformer has been developed.
FIG. 1 is a circuit diagram illustrating an example of a conventional VCO using a transformer. In the VCO, a core unit of the VCO is coupled to a transformer 11 having two pairs of terminals.
The conventional VCO comprises an LC tank 12 and the core unit. The LC tank 12 includes varactor diodes C1 and C2 coupled between one pair of terminals of the transformer 11, and varactor diodes C3 and C4 coupled between the other pair of terminals, the varactor diodes C1 to C4 being for tuning an oscillation frequency. The core unit further includes transistors Q11 to Q15, gates and drains of some of the transistors are coupled to the pair of terminals.
In the voltage control oscillator, an oscillation frequency is tuned by the varactor diodes C1 and C2 coupled between the pair of terminals of the transformer 11 and the varactor diodes C3 and C4 coupled between the other pair of terminals of the transformer 11. In addition, a mutual inductance increases and thus the Q factor is improved.
FIG. 2 is a circuit diagram illustrating an example of a conventional low noise amplifier (LNA) using a transformer. The LNA has a differential structure converting a single input received through one pair of terminals of a transformer 21 into a differential input.
An RF signal is input through one pair of input terminals of the transformer 21, and a pair of output terminals of the transformer 21 are respectively coupled to an output unit 22 including two serially coupled transistors Q21 and Q22 and an output unit 23 including two serially coupled transistors Q23 and Q24.
In the LNA, since a mutual inductance decreases, inductances of the respective terminals should be exactly estimated to adjust inductance matching therebetween.
In general, a Q factor of an inductor is represented by the following Formula 1:
                              Q          =                                                    ω                0                            ·              L                        R                          ,                            Formula        ⁢                                  ⁢        1            where L is an inductance and R is a resistance.
In this manner, the Q factor of the inductor is determined by the inductance L and the resistance R. The conventional art using the transformer as described above increases only a mutual inductance. This only improves the Q factor but does not significantly increase it because an equivalent resistance component of the transformer itself is kept unchanged. That is, an improvement range of the Q factor is limited.